Introduction

The coastal zone often experiences a unique weather which results into a very special climate. Coastal meteorology is the study of meteorological phenomena within about 100 km inland or offshore of a coastline. Improved understanding of the processes in the meteorology of the coastal zone is based on detailed knowledge of marine and terrestrial boundary layers and air-sea-interaction but has also to consider large-scale atmospheric dynamics and circulation of the coastal ocean. In addition to the importance of coastal meteorology to coastal weather forecast the subject helps in understanding the physical, chemical and biological aspects of the coastal ocean. Furthermore, the application of the knowledge is vital for the prediction of sea state and pollutant dispersal, and it is also important for public safety, ship routing and naval operations.
The phenomena in coastal meteorology are caused, or significantly affected, by sharp changes in heat, moisture, and momentum transfer and changes in elevation, often a complex orography, that occur between land and water. Thermally driven effects like the land-sea breeze and orographically induced flows are the most prominent features in coastal meteorology, but also coastal cloud systems and fog, low level jets, coastal fronts and land-falling hurricanes, whose low-level flows are often modified as to favour the formation of tornadoes, are aspects of coastal weather phenomena. Complex terrain or coastlines and marine boundary layer stratus complicate the subject of coastal meteorology.

Structured coastline

Boundary Layer Processes, Air-Sea Interaction aspects

The Atmospheric boundary layer (ABL) is the lowest layer of the atmosphere which most directly is influenced (on time scales of an hour or less) by the presence of the ground. Turbulent motions dominate the flow in this region. Strong momentum, heat, water vapour, trace gas and particle transfer occurs at the air-land and air sea interface which is mainly driven by turbulent motions Garratt, 1995 [1]). The ABL is the region where life and human activities predominantly take place (Arya, 2001)[2]

Thermally driven effects, the land-sea breeze

There is generally a large thermal contrast between the ocean and the land that drives the well-known sea-breeze circulation, which results in the confluence of air originating over the ocean with air originating over the land. The sea-breeze is associated with many processes that contribute to the recirculation and trapping of pollution, the evolution of precipitating convective storms, the creation of strong nearshore thermal, moisture and aerosol gradients, and the formation and transport of fog and low cloud in the coastal zone.

Qualtitative description of a sea breeze mechanism in a calm atmosphere during a clear day:
- air over land heats up and expands more rapidly than that over water
- vertical pressure gradient larger in cooler air over water (hydrostatic cond.)
- there is a level where pressure is higher over land than over water
- pressure gradient produces slight flow from B to C
- convergence near C leads higher pressure and subsidence from C to D
- departure from hydrostat. equi. around D and flow from D to A (sea breeze)
- divergence near B leads to decrease in pressure and flow from A to B

Simplified sea-breeze-circulation during day.

Qualtitative description of a sea breeze mechanism in a calm atmophere during a clear night:
- at night the land cools more rapidly than the sea
- at upper levels pressure relatively high over sea and low over land
- there is a level where pressure is higher over land than over water
- pressure gradient produces slight flow from C to B
- convergence near B leads to higher pressure and subsidence from B to A
- departure from hydrostat. equi. around A and flow from A to D (land breeze)
- divergence near D leads to decrease in pressure and flow from D to C

Simplified sea-breeze-circulation during night.

Mechanically induced flows

Coastally-trapped disturbances that exist for between two and six days, can have length scales of 1000 km in the alongshore direction and 100-300 km across-shore.
These features generally cause significant changes in the local weather in the coastal zone, for example, replacing clear skies with clouds and fog, and causing intensification and reversals of the wind field as the system moves along the coast.

Orographic Influences

Coastal mountains form a barrier to the wind field that may affect both the downstream and upstream evolution of the flow. The problem is characterized by two free parameters, the Froude number Fr, defined by U/(Nhm) and the Rossby number Ro, defined by U/(flm), where U is the speed of the air stream, hm is the height of the barrier, f is the Coriolis parameter, lm is the half width of the barrier, N is the Brunt-Vaisala frequency and is equal to (g/q0 dq/dz)0.5, g is gravity, and q0 is the mean potential temperature (the temperature of a parcel of air moved dry adiabatically to a pressure of 1000 mb). Generally blocking of the air flow occurs when Fr is < 1, which for a typical value of N of 10-1 s-1 can occur with elevations as low as 100 m. The influence of the earth's rotation on the deceleration of the upstream flow is considered through Ro. Deceleration is insignificant when Ro < 1. In steep topography it has been shown that the deceleration zone will grow upstream to a width defined by the Rossby radius of deformation lr, which is equal to Nhm/f. Steep topography is defined by the non-dimensional slope (hm/lm)(N/f), being greater than 1. This may also be written as Ro/Fr. In the coastal region the mountains often represent a wall such that lr is typically greater than lm, where Ro > 1, and the flow is not expected to be geostrophic (i.e., the flow will not remain perpendicular to the pressure gradient). The offshore influence of mountain coasts is given by the Rossby radius of deformation, which typically varies from 10 to 100 km.
When uniform onshore flow, characterized by a low Froude number, encounters a coastal barrier the steady-state response is a pressure ridge, a phenomenon referred to as damming. The topographically induced pressure fields produce along-ridge pressure gradients that can result in barrier jets. The best examples of the phenomenon are found associated with cold air damming between a coastal front and mountain ridges. A similar structure can occur when the incident flow is not uniform, such as in the vicinity of a storm.

Coastal stratocumulus (Photo: M. Quante.

Interactions with large scale meteorological systems

Many of the meteorological phenomena that are associated with the coast are a consequence of synoptic-scale forcing.Traveling disturbances as they pass over the coastline can be strongly modified and lead to new phenomena that are peculiar to the coastal region. Land-falling storms are directly modified in the coastal region where changes in the bottom boundary conditions are dramatic.
Severe storms affect many coastal regions during the winter and, to a lesser extent, in summer. Regional differences exist, with oceanic extratropical cyclones affecting the coast and continental extratropical cyclones moving from the interior of the continents tothe coast. Tropical cyclones, including hurricanes, may affect almost any part of the coasts in the tropics during summer and fall. Particularly important effects are caused e.g. by cold air outbreaks along the east coast of the United States and by rapidly intensifying storms over the Gulf Stream.
Hurricanes/typhoons weaken over land because of the absence of latent heating at the surface.
Orographic barriers can affect the dynamics of the storm through blocking and actually enhance precipitation and winds in coastal areas. Increased friction over the land decreases the surface winds, which cause an expansion of the radius of the eye wall. This can cause an increase in the vertical wind shear, which may explain the occurrence of tornadoes in these storms.
Many of the meteorological phenomena that are associated with the coast are a consequence of synoptic-scale forcing; for example, the formation of coastally-trapped Kelvin waves and low level frontogenesis caused by large gradients in surface temperature. The response time of the coastal currents are sufficiently short that changes in the shelf circulation can be driven by rapidly moving atmospheric events.
The formation of intense storms along coastlines or ice edges at high latitudes have been well documented. These appear to form over water just beyond the ice edge, where the large vertical temperature gradient between the water and the air leads to strong low level baroclinicity. It has been shown that some polar lows can attain the intensity and structure commonly associated with hurricanes.

Meteorological measurements in the coastal environment

Understanding the coastal atmosphere requires a multidisciplinary approach to combine research on air motions, cloud physics, aerosol dynamics, convection, air-sea gas exchange, surface waves and fluxes, boundary layer dynamics, and large scale storm systems to investigate interactions and feedbacks between these processes. Therefore the suite of meteorological instruments and well organized measurement strategies are needed to capture the the relevant parameters of the coastal atmosphere and air-water interface.

Off-shore turbulent flux measurements.

The small space and time scales associated with the coastal zone place severe demands on measurement systems. Space-borne remote sensing systems have the potential to measure phenomena both over the coastal ocean and over the land; however, besides routine photogrammetry of clouds and infrared imagery, satellite data from rcently launched systems starts to provide the required spatial resolution.